AE438
GPS Accuracy for Tree Scouting and Other Horticultural
Uses1
Reza Ehsani, Sherrie Buchanon, and Masoud Salyani2
Introduction
After the outbreak of citrus greening disease
(HLB), tree scouting has become a normal practice in
Florida. In most cases, growers are using global
positioning systems (GPS) to map the infected trees
for subsequent tree removal and recordkeeping
purposes. While GPS-located maps are somewhat
accurate, there are instances in which the GPS
location could be significantly inaccurate. This could
then cause problems locating the infected trees or
using the location data for future use. Figure 1 shows
an example of a GPS location map of HLB-infected
trees (red triangles) that are overlaid on an aerial
image. As shown, it is impossible to determine from
some data points (circled in blue) the correct HLBinfected tree. This article provides some simple
explanations of the causes of GPS error and the level
of accuracy that can be expected from different
classes of GPS receivers.
Figure 1. A GPS location map of HLB-infected trees
overlaid on an aerial image.
How GPS works
The GPS receiver, like a radio, receives signals
from different GPS satellites. Every time the GPS
receiver receives a signal, it calculates its distance
from that satellite. When there are at least four
satellites in view, the GPS receiver can calculate its
exact location (i.e. latitude, longitude and altitude)
using triangulation. GPS constellation and the
satellite orbits are designed in such a way that for any
1. This document is AE438, one of a series of the Agricultural and Biological Engineering Department, Florida Cooperative Extension Service, Institute of
Food and Agricultural Sciences, University of Florida. Original publication date January 2009. Revised October 2009. Visit the EDIS Web Site at
http://edis.ifas.ufl.edu.
2. Reza Ehsani, assistant professor; Sherrie Buchanon, senior engineering technician; Masoud Salyani, professor; Department of Agricultural and Biological
Engineering, Citrus Research and Education Center (REC)-Lake Alfred; Florida Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville, FL 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. 0LOOLH
)HUUHU&KDQF\,QWHULP'HDQ
GPS Accuracy for Tree Scouting and Other Horticultural Uses
point on earth at any particular time there are at least
four satellites in view at ground level. In most cases,
there are 5-9 satellites in view. In the near future, this
number will be increased because European countries
are putting a similar system, called Galileo, into orbit.
The Galileo system will use the same signal structure
as GPS satellites which means that in the future, there
will be more satellites in view in the sky. With more
satellites in view, signal accuracy is expected to be
improved, which should benefit GPS orchard
applications.
Sources of Error
There are many factors that affect GPS accuracy.
Some of them are caused by the slight deviation of
GPS satellites from their orbit or by the atmospheric
and environmental conditions through which the
signals must travel to reach the GPS receiver. The
geometry of satellite position in the sky, i.e., where
satellites are located at any moment, can also affect
GPS accuracy. Let's assume there are five satellites in
view and they are evenly distributed in the sky. This
configuration of satellites could produce much
smaller error compared to a situation in which five
satellites are clumped near each other in one region of
the sky. The geometric orientation of the GPS
satellites is referred to as “dilution of precision”
(DOP) and can influence GPS error with changes in
time and location. Some GPS receivers can show the
DOP value. A DOP value between 2 and 4 is
considered good and anything above 8 is considered
to be poor and could cause high GPS error.
Differential Correction Signal
To reduce error, GPS receivers can acquire other
radio frequency signals called "differential
correction" signals. There are several types of
differential correction signals available today; some
of them are free while others must be obtained by
paying a subscription fee. The two most common
sources of free differential signals are "wide area
augmentation system" (WAAS) and "coast guard
beacon" (CGB) signals. Almost all GPS receivers
available on the market today have the capability of
using the WAAS signal. The CGB radio frequencies
are susceptible to noise and interference. To use the
CGB signal, the receiver needs additional hardware;
2
therefore, these receivers are usually costlier and a
little larger in size. Nowadays, most new receivers
that can receive the CGB signal can also receive the
WAAS signal. The advantage of these receivers is
that if they lose one correction signal, they can
automatically switch to the other correction signal to
maintain higher GPS accuracy and resolution.
Accuracy Classifications
GPS receivers are typically designed for
different purposes, which usually corresponds to the
level of accuracy they possess. The level of accuracy
required depends on the user and application and can
be expressed in terms of relative and absolute
accuracy. Relative accuracy is a measure of
short-term repeatability of a GPS receiver. Absolute
accuracy determines how close the reported location
is to its true location. A good GPS receiver has a good
absolute and relative accuracy. Some of the lower
cost receivers have good relative accuracy but only
fair absolute accuracy. If you plot the data from such
a receiver, you will notice that the datapoints are very
good with respect to each other; however, they are
shifted from their true position as illustrated in Fig.
1.
What to consider when choosing a
GPS receiver for scouting
Knowing the level of accuracy needed for a
particular application is important in selecting the
right GPS. A GPS user needs to understand the
limitations of different types of receivers and how
those limitations will affect the data collected. The
currently available GPS receivers used for
agricultural applications can be classified into four
general categories based on their accuracy level. The
accuracy levels are less than 15 feet, less than 3 feet,
less than 12 inches, and less than 1 inch horizontally.
The first two levels (<15 feet and <3 feet) are
primarily useful for soil sampling and scouting type
applications. The remaining two levels (<12 inch and
<1 inch) describe applications pertaining to vehicle
guidance and surveying. Table 1 shows the price
range and typical applications for these four classes
of GPS receivers. The highest level of accuracy can
be obtained by “Real Time Kinematic” (RTK)
GPS. This type of GPS works on a different principle
GPS Accuracy for Tree Scouting and Other Horticultural Uses
compared to regular DGPS receivers. RTK GPS are
usually very expensive and mainly used for
surveying. However, recently they have become
increasingly utilized for certain agricultural
applications such as auto-steering guidance.
Things to consider when purchasing
a GPS Receiver
• When comparing GPS receivers, make sure the
units of accuracy are comparable. A GPS with 6
ft RMS position accuracy has the same accuracy
level as a GPS with 12 ft 2DRMS.
• In general, if you are close to a Coast Guard
Beacon signal (less than 50 miles) you can
expect to get less errors compared to WAAS.
However, in central Florida, errors from WAAS
and Coast Guard Beacon signals are nearly
equivalent.
• Some low-cost GPS receivers only use
6-decimal places to report coordinate
information compared to the 8-decimal places
that higher-end GPS receivers provide. This
small fractional difference can translate to large
differences in GPS receiver accuracy.
• Tree canopy can have a mixed effect on GPS
accuracy depending on the orientation, tree size,
and density. In general, for a very tall and dense
canopy, the antenna mounting height should be
raised to a level near the top of the canopy to
avoid signal blockage.
• Keep in mind that accuracy performance of
GPS is good 95% of the time but there is still a
5% chance that you may acquire a point outside
the machines accuracy range.
• Before purchasing a GPS, first decide what
level of accuracy you need. Keep in mind that
you generally pay more to achieve higher
accuracy.
• Most commonly available GPS receivers have
12 channels which means they can track up to 12
satellites at the same time. Since, there are going
to be more satellites in view in the sky at any
given time, in the near future it would be a good
idea to purchase a GPS with greater than 12
3
channels. There are some new GPS systems that
have 20 channels.
• If you need to go back to a specific field
location year after year, you should consider
RTK because the position of a recorded point
with a regular differential global positioning
system (DGPS) receiver has the tendency to
shift with time, but not with RTK.
• If you have already purchased a GPS that has a
low level of accuracy, make sure to use other
means of marking the infected tree (paint, etc.)
and don't just trust the GPS location
information.
Useful information on Specific GPS
Receiver specifications
Trimble Ag 106 – In differential GPS mode
using WAAS DGPS corrections, the AgGPS 106
outputs positions with 1-3 m (3–10 ft) accuracy
(static HRMS). Connects to laptop/PDA using a
serial connection.
Trimble Ag 332 – It has +/- 2.5 cm (1 in)
pass-to-pass and year-to-year repeatable accuracy
using RTK, +/- 5 – 10 cm (5 – 10 in) pass-to-pass
accuracy using OmniSTAR HP, +/- 7.5 – 12.5 cm (3
– 5 in) pass-to-pass accuracy using OmniSTAR XP,
+/- 15 – 20 cm (6 – 8 in) pass-to-pass accuracy
using WAAS, EGNOS, beacon, or OmniSTAR
VBS.
Trimble Ag 214 – RTK - 1 cm (0.4 in) + 2 parts
per million (ppm) baseline distance horizontal 2 cm
(0.79 in) + 2ppm baseline distance vertical.
Trimble Ag 252 – Horizontal RTK positioning
accuracy 2.5 cm (0.98 in) + 2 ppm, 2 sigma; vertical
RTK positioning accuracy 3.7 cm (1.5 in) + 2 ppm, 2
sigma Sub-meter differential accuracy (RMS),
assuming at least five satellites and a PDOP of less
than four.
Garmin eTrex Legend – Position < 15 m (< 50
ft), 95% typical, Velocity 0.05 m/s steady state.
WAAS Accuracy: Position: less than 3 m (10 ft)
RMS Velocity: 0.1 knot RMS steady state.
GPS Accuracy for Tree Scouting and Other Horticultural Uses
Garmin GPS 72 - GPS Accuracy: Position < 15
m (< 50 ft), 95% typical, Velocity: 0.05 m/s steady
state.
Garmin GPS 17 – Position: < 15 m, 95%
typical, Velocity: 0.1 knot RMS steady state, DGPS
(USCG) accuracy: Position: 3-5 m (9.84 – 16.4 ft),
95% typical, Velocity: 0.1 knot RMS steady state,
DGPS (WAAS) accuracy: Position: < 3 m (< 9.84
ft), 95% typical, .1 knot RMS steady state.
Garmin 18 GPS - In differential GPS mode using
WAAS DGPS corrections, the Garmin 18 GPS
outputs positions with 1-3 m (3 – 10 ft) accuracy
(static HRMS). Connects to laptop/PDA using either
USB or Serial connection.
TopCon GMS-2 – Handheld, 3mm (0.3 in) +0.8
ppm horizontal, 4mm (0.4 in) + 1.0ppm vertical
(static), 50cm (1.64 ft) HRMS Post Processing, 50cm
(1.64 ft) HRMS Real time (DGPS), 10mm (4 in) +
1ppm horizontal, 15mm (5.9 in) + 1ppm vertical (PP
Kinematic), 50 channels cold tracking - <30 seconds,
warm tracking - <10 seconds, GPS/GLONASS L1
C/A Code & Carrier WAAS/EGNOS/MSAS.
TopCon GR3 – Static 3mm (0.3 in) +.5ppm
horizontal, 5mm (0.5 in) + 0.5 ppm vertical,
RTK/Kinematic 10 mm (1 in) + 1 ppm horizontal,
15mm (1.5 in) + 1 ppm vertical, DGPS > 0.3m (> 0.8
ft) Post Processing, < 0.50m (< 1.6 ft) Real time.
Magellan MobileMapper CE – Real-time
accuracy – sub-meter, Post-mission accuracy - < 30
cm (6.4 ft), Bluetooth, USB host and slave, RS232,
Waterproof to one meter depth for 30 minutes.
Magellan MobileMapper Pro – Real-time
accuracy – 2-3 m (6.6 – 9.9 ft), Post-mission
accuracy – Sub-meter, Connector – Serial data
cable, Waterproof to one meter depth for 30 minutes.
Lowrance iFinder Expedition C – WAAS and
GPS Accuracy to 7 m (23 ft), Waterproof to IPX7
standard, Built-in microphone for recording voice
notes to waypoints, Built in Compass and barometric
altimeter with weather predictions.
AgLeader GPS 5100 – Differential position
accuracy – less than one m (3.8 ft) horizontal is all
of the following criteria are met: At least five
satellites, PDOP <4, RTCM SC-104 corrections,
Standard format broadcast from a Trimble MS750,
AgGPS 214, or equivalent reference station. RTK
position accuracy 2.5 cm (0.98 in) + 2 ppm, 2 sigma,
and vertical 3.7 cm (8.3 in) + 2 ppm, 2 sigma.
AgLeader GPS 1100 – Accuracy with
WAAS/EGNOS Diff: 2m (6.6 ft) RMS, Accuracy
GPS only: 4m (13.1 ft) RMS, Output NMEA-0183.
Trimble Juno – Handheld GPS with 2 to 5 m
(6.6 – 16.4 ft) accuracy WAAS.
Raven Phoenix 200 smart antenna – up to 10
Hz, WAAS, 2 – 5 m (6.6 – 16.4 ft) accuracy.
Raven Invicta 210 – up to 10 Hz, WAAS and
Beacon Corrected.
Raven Invicta 100 – 2 m (6.6 ft) RMS,
WAAS.
Raven Invicta 115 – Smart Antenna no separate
receiver box, Differential Correction Option: WAAS,
10 Channel, 5 Hz update rate, 2 serial port Accuracy
<1m (3.3 ft) RMS.
Raven Invicta 310 – Integrated high
performance GPS receiver with WAAS, radiobeacon
and L-Band satellite differential.
Raven Invicta 410 – Decimeter differential GPS
accuracy, Data rate up to 20 Hz, L1/L2 capable,
WAAS, CDGPS, OmniStar, EGNOS and MSAS;
Decimeter accuracy: OmniSTAR HP & Raven
Enhancer.
Raven Phoenix 300 smart antenna – up to 10
Hz, WAAS, Canadian WAAS, and OmniStar VBS,
Sub meter accuracy and upgradeable to decimeter
accuracy.
TeeJet RX370p – up to 10 Hz, WAAS, EGNOS
and e-diff sub meter accuracy.
TeeJet RX500 – user configurable up to 20 Hz,
5-10 cm (2– 4 in) accuracy.
TeeJet RX410p – WAAS, Omnistar, beacon and
e-diff sub-meter accuracy, up to 10 Hz update rate.
4
GPS Accuracy for Tree Scouting and Other Horticultural Uses
HP iPaq rx5915 – Handheld GPS WAAS,
WLAN (802.11b/g), Bluetooth 2.0 Enhanced Data
Rate, Samsung SC32442 400MHz Processor,
Integrated SiRFstarIII GSC3F GPS.
Mio DigiWalker P550 – Handheld GPS WAAS,
20 channel SiRFStarIII GSC3F GPS, Samsung 2440
400 Mhz processor.
Manufacturers
Ag Leader Technology, Inc. –Tel:
(515)232-5363 website: www.agleader.com
Garmin International Inc. –Tel: (913)397-8200
website: www.garmin.com
Lowrance Electronics, Inc. –Tel:
1-800-324-1356 website: www.lowrance.com
Magellan Navigation, Inc. –Tel:
1-800-707-9971 website: www.magellangps.com
Topcon America Corp. –Tel: 1-800-223-1130
website: www.topconpa.com
Trimble Navigation Limited –Tel:
1-800-874-6253 website: www.trimble.com
Teejet - Tel: 630-665-5000 website:
www.teejet.com
Raven –Tel: (800)243-5435 website:
www.ravenprecision.com/
HP –Tel: (800)474-6836 website: www.hp.com
Mio – Tel: (866)-646-4477 website:
www.mio-tech.com,
Useful links
GPS Tutorial http://www.trimble.com/gps/index.shtml
GPS Overview http://www.colorado.edu/geography/gcraft/notes/gps/
gps_f.html
GPS Tutorial http://facility.unavco.org/project_support/campaign/
training/jpstutorial.pdf
5
GPS Accuracy for Tree Scouting and Other Horticultural Uses
6
Table 1. Comparison of four classes of GPS receivers
Free DGPS Based
Subscription
Real Time Kinematic (RTK)
Accuracy
Level
5 - 15 ft
1.52 - 4.57 m
2 - 3 ft
.61 - .91 m
10 - 12 in
1 - 1.2 cm
Local Base
1 - 2 in
10 - 20 mm
CORS
1 - 2 in
10 - 20 mm
Price
$100 - $800
$800 - $2500
$2500 - $6000
Base: $16k $25k Rover:
$16k - $20k
Differential
Correction
Source
WAAS
Dual Frequency
StarFire 1
OmniStar XP & HP
StarFire 2
RTK Base
Base: ~$30k
Rover: $6k $12k Yearly
Fees: ~$1k
Local Network of
RTK Base
Stations
Static
Accuracy
3 - 5 Feet
12 - 36 Inches
8 - 12 Inches
1 - 2 Inches
(Determined
by Distance to
Base Station)
1 - 10 Inches
Static
Stability
Not Stable
Fairly Stable
Stable
Very Stable
Very Stable
Year to Year
Repeatability
None
None
Marginal ~ 1 Foot
Very Stable
Very Stable
Precision Ag
Uses
Soil
Sampling
Previous +
Non-Critical
Lightbar
Guidance
(Spraying,
Tillage) Yield
Monitoring
Previous + Critical
Lightbar Guidance
(Planting)
Previous +
Autosteering
Cultivating
CORS uses a
cellular modem
to transmit
correction
information. It is
mainly used by
surveyors, but is
gaining interest
in agriculture.
GPS Accuracy for Tree Scouting and Other Horticultural Uses
7
Table 2. Comparison of some commercially available GPS receivers in four accuracy classes
Class I
Class II
Class III
Class IV
Trimble Juno
$650*
Raven Invecta 115
$1,245*
TopCon GMS-2
$4,350*
Trimble Ag 214
$20,000 - $30,000*
Garmin 18
$75*
TeeJet RX370p –
$1,844*
AgLeader GPS 5100
$5,000*
Trimble Ag 252
$20,000 - $30,000*
Garmin 72
$99 - $200*
Trimble 106
$1,200*
Raven Invicta 310
$3,140*
TopCon GR3
$33,845 - $63,119*
Garmin 17
$130*
Magellan Mobile Mapper CE
$1,975 - $2,500*
Raven Invicta 410
$5,750*
Garmin e-trex legend
$130 - $250*
Magellan Mobile Mapper Pro
$1,750 - $2,750*
Raven Phoenix 300 smart
antenna
$3,660 - $5,485*
Lowrance iFinder
Expedition C
$242 - $350*
Raven Phoenix 200 smart
antenna
$1,465*
TeeJet RX500 - $5,154*
HP iPaq rx 5915
$500*
Raven Invicta 210
$2,005*
TeeJet RX410p
$4,151*
Mio-Tech P550
$289 - $339*
AgLeader GPS 1100
$895*
Raven Invicta 100
$645*
*This is estimated retail price for 2008 and is subject to change.
Trimble 332
$3,000*